As is known, head boxes are used to form a web consisting of at least three layers on a paper machine or similar. It is usually meaningful to manufacture the various layers with different properties. Such properties are for example the type or nature of the fiber or also other parameters such as consistency, fine particle content or similar. The head box used for this, has, among other things, the task of producing a wide jet which then subsequently enters for instance into the forming region of a paper machine. For this, it is expedient to provide a pressure channel, also called a nozzle volume, in the head box which opens out into a slit for each of the suspension streams. Pressure energy is converted into movement energy during the flow of material suspension through the channel/nozzle and on emission from the slit, i.e. the suspension is accelerated considerably.
In the known trilayer and multilayer head boxes, the individual partial streams in the jet converge at a particular angle. This angle is largely dependent on the construction parameters of the head box, for instance the spatial requirement of material supply and material distribution and of the positioning devices. On convergence of the various suspension partial streams in the jet, the further outer lying partial streams also have, as a result of the angle, a speed component at right angles to the direction of a neighbouring partial stream. As a result of these speed components, flow momentum exists in this transverse direction which amplifies the intermixing of neighbouring layers. The degree to which this mixing occurs is undesired. After all, such a multilayer head box is considerably more expensive than a single layer head box not only in its manufacture but also in operation and the benefit of this expense is reduced if an adequate separation of the various partial streams in the sheet is not possible.
A guiding which is, as far as possible, parallel--i.e. at an acute angle--of the partial streams in the head box is conceivable and sometimes practised. The undesired lateral speed components can thereby be kept small but on the other hand the head box becomes even larger hence longer and still more expensive.
In other known head boxes with a short construction the partial streams are already diverted in the head box so that they can be emitted almost parallel. This can lead to separation and secondary streams forming in the pressure channels of the outer layers where the diversion takes place which are detrimental to the jet uniformity.